An LTE (Long Term Evolution) system is in practical use as one of technologies used for a wireless access network of a mobile communications system. Handover in this LTE system will be described simply.
FIG. 10 is a figure for explaining a communication network including a related LTE system.
The wireless access network is connected to a core network. In the wireless access network, eNBs (evolutional Node Bs) 101 to 103 that are the base stations corresponding to the LTE system are installed. In the core network, MMEs (Mobile Management Entities) 21 and 22 that accommodate the eNB and are the devices of a core network system are installed.
In FIG. 10, an inter-station link that is a communication link between an eNB 102 and an eNB 103 is established. This inter-station link is called an “X2 interface” or an “X2 link” and hereinafter, referred to as the “X2 link”.
In an example shown in FIG. 10, a core-inter station link that is a communication link between a MME 21 of a core network side and the eNB 101 is shown. This core-inter station link is called an “S1 interface” or an “S1 link” and hereinafter, referred to as the “S1 link”. In FIG. 10, the S1 link is established between the MME 21 and the eNB 102, between the eNB 102 and the MME 22, and between the eNB 103 and the MME 22. Further, FIG. 10 shows a state in which a UE (User Equipment, a portable terminal) 31 moves from a communication cell of the eNB 102 to a communication cell of the eNB 101 and a UE 32 moves from a communication cell of the eNB 102 to a communication cell of the eNB 103.
FIG. 11 is a sequence diagram showing a handover procedure in which a handover is performed between the eNBs shown in FIG. 10. Here, as shown in FIG. 10, a case in which the X2 link is established between the eNBs (between the eNB 102 and the eNB 103) will be explained.
In the handover performed between the eNBs, the eNB 102 transmits a handover request (HANDOVER REQUEST) to the eNB 103 and receives a handover request confirmation (HANDOVER REQUEST ACKNOWLEDGE) from the eNB 103 (refer to non-patent literature 1).
In a case in which the X2 link is established between the eNBs, as described above, when the UE performs the handover, information is transmitted and received between the eNBs by using the X2 link and whereby, the handover can be performed. When the X2 link is not established between one eNB and another eNB that is a handover destination, two eNBs have to transmit/receive the information via the S1 link. In this case, the time required to perform handover increases and the burden on the MME of the core network side increases.
A SON (Self-Organizing Network) function is studied in 3GPP (3rd Generation Partnership Project) and an ANR (Automatic Neighbor Relation) function is included in the SON function (refer to non-patent literature 2). The SON function is a function to automatically and optimally set a parameter of the base station. The purpose of the ANR function is to reduce the burden on an operator which occurs at the time of managing the relationship between the eNBs that are adjacent to each other. The ANR function manages another eNB that exists in the neighborhood, in other words, the adjacent (Neighbor) eNB, by using a NRT (Neighbor Relation Table) and adds the adjacent eNB in the NRT when the adjacent eNB is detected. When the eNB detects a new adjacent eNB by this ANR function, it is expected to establish the X2 link between the eNB and the adjacent eNB.
However, in an actual network, the number of the X2 links which can be established by one eNB is limited because there is a limitation on installation. Therefore, it is impossible for the eNB to establish the X2 link with all the eNBs detected by the ANR function without limitation. For this reason, when the number of the established X2 links is equal to the maximum installable number of the X2 links, the X2 link cannot be newly established. As a result, handover is frequently performed via the S1 link and the burden on the core network increases. Further, when an EMS (Element Management System) for managing the eNB is installed, if the useless X2 link remains established, the burden on the EMS increases.
An example of a method for removing the useless X2 link is disclosed in patent literature 1. In the invention disclosed in patent literature 1, a source eNB acquires the number of occurrences of handover to a target eNB and compares it with a predetermined threshold value. When a comparison result shows that the number of handover occurrences is less than the predetermined threshold value, the source eNB determines that the number of occurrences of handover to the target eNB decreases and releases the X2 interface between the source eNB and the target eNB.
Further, in patent literature 2, it is disclosed an invention in which when handover is performed between the eNBs that are adjacent to each other, a user (U-Plane) signal and a control (C-Plane) signal are directly forwarded via the X2 interface. Further, a second base station receives a broadcast packet from a first base station, determines an adjacent condition between wireless service areas based on location information and radius information of a wireless service area included in a received packet and location information and radius information of the wireless service area of the first base station and establishes the link between the first and second base stations that satisfy this adjacent condition.